Photonic multiplexer for single-photon sources

ABSTRACT

A device includes a plurality of photon sources coupled to a plurality of output terminals. The plurality of photon sources are coupled together, by a first switch layer, into a plurality of photon source groups. The first switch layer comprises a plurality of switches. The device further includes a second switch layer coupled to output terminals of the first switch layer. The second switch layer includes a plurality of second layer n-by-n switches and a plurality of second layer l-by-l switches, wherein l is less than n. At least two output terminals from two respective photon sources residing within a first photon source group of the plurality of photon source groups and a second photon source group of the plurality of photon source groups are coupled directly to respective output terminals of the device without being coupled to any intervening second switch from the second switch layer.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/455,534, filed Jun. 27, 2019, now U.S. Pat. No. 10,802,222, which isa continuation-in-part of U.S. application Ser. No. 16/231,022, now U.S.Pat. No. 10,677,985, filed Dec. 21, 2018, entitled “Photonic Multiplexerfor Single-Photon Sources,” which claims priority to U.S. ProvisionalApplication 62/609,287, filed Dec. 21, 2017, entitled, “PhotonicMultiplexer for Single-Photon Sources,” each of which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

This relates generally to photonic devices (or hybridelectronic/photonic devices) and, more specifically, to photonic devices(or hybrid electronic/photonic devices) that multiplex photons fromprobabilistic single-photon sources.

BACKGROUND

Single-photon sources are light sources that can emit light as singleparticles (photons) at respective times. These sources are useful in awide variety of applications. However, single-photon sources do notbehave deterministically. That is, for each attempt to emit asingle-photon, the probability of success is less than 100%, meaningthat, sometimes, no photon is emitted at all for a particular attempt.In some circumstances, an attempt to produce a single-photon may producetwo photons, which may also be considered an unsuccessful attempt.Accordingly, there is a need for methods and devices that improve theefficiency and reliability of single-photon sources (e.g., theprobability of producing a single-photon).

SUMMARY

Efficient, reliable single-photon sources are important for applicationsin quantum computing where there is a need to produce well-defined (orsomewhat-well-defined) entangled states of photons.

The above deficiencies and other related problems are reduced oreliminated by photonic multiplexers described herein. The multiplexersdescribed herein, when combined with a redundant array of such sourcesproduce an output that, effectively, has the characteristics of asingle-photon source with a much higher efficiency and reliability(e.g., single-photon generation success rate) than individualsingle-photon sources. The multiplexers are also capable of routing thesingle-photons to preselected channels, thereby further improving theutility of the combined devices.

One or more embodiments of the present disclosure provides a device formultiplexing photons (e.g., a photonic multiplexer). The device includesa plurality of first switches. Each first switch in the plurality offirst switches includes a plurality of first channels. Each first switchis configured to shift photons in the plurality of first channels byzero or more channels, based on first configuration information providedto the first switch. The device further includes a plurality of secondswitches, each second switch in the plurality of second switchesincludes a plurality of second channels, each second switch including arespective second channel coupled with a respective first channel from adistinct first switch of the plurality of first switches. Each secondswitch is configured to shift photons in the plurality of secondchannels by zero or more channels, based on second configurationinformation provided to the second switch.

Further, one or more embodiments of the present disclosure providesanother device for multiplexing photons (e.g., a photonic multiplexer).The device includes a first switch coupled with a first channel and asecond channel. The first switch is configured to shift photons by zeroor more channels based on first configuration information provided tothe first switch, including (i) maintaining a photon in the firstchannel and maintaining a photon in the second channel when the firstconfiguration information indicates shifting by zero channels and (ii)shifting the photon in the first channel to the second channel andshifting the photon in the second channel to a channel that is distinctfrom the second channel when the first configuration informationindicates shifting by one channel.

The device includes a second switch coupled with a third channel and afourth channel. The second switch is configured to shift photons by zeroor more channels based on second configuration information provided tothe second switch, including (i) maintaining a photon in the thirdchannel and maintaining a photon in the fourth channel when the secondconfiguration information indicates shifting by zero channels and (ii)shifting the photon in the third channel to the fourth channel andshifting the photon in the fourth channel to a channel that is distinctfrom the fourth channel when the second configuration informationindicates shifting by one channel.

The device includes a third switch coupled with the first channel andthe third channel. The third switch is configured to shift photons byzero or more channels based on third configuration information providedto the third switch, including (i) maintaining a photon in the firstchannel and maintaining a photon in the third channel when the thirdconfiguration information indicates shifting by zero channels and (ii)shifting the photon in the first channel to the third channel andshifting the photon in the third channel to a channel that is distinctfrom the third channel when the third configuration informationindicates shifting by one channel.

The device includes a fourth switch coupled with the second channel andthe fourth channel. The fourth switch is configured to shift photons byzero or more channels based on fourth configuration information providedto the fourth switch, including (i) maintaining a photon in the secondchannel and maintaining a photon in the fourth channel when the fourthconfiguration information indicates shifting by zero channels and (ii)shifting the photon in the second channel to the fourth channel andshifting the photon in the fourth channel to a channel that is distinctfrom the fourth channel when the fourth configuration informationindicates shifting by one channel.

Further, one or more embodiments of the present disclosure provides amethod of multiplexing photons. The method is performed at a device thatincludes a plurality of first switches (e.g., a first switching layer)and a plurality of second switches (e.g., a second switching layer).Each first switch in the plurality of first switches includes aplurality of first channels. Each second switch in the plurality ofsecond switches includes a plurality of second channels. Each secondswitch includes a respective second channel coupled with a respectivefirst channel from a distinct first switch of the plurality of firstswitches.

The method includes, at a first switch, receiving a first set of photonsin the plurality of first channels and shifting photons in the set ofphoton in the plurality of first channels by zero or more channels,based on first configuration information provided to the first switch.The method further includes, at a second switch, receiving a second setof photons in the plurality of second channels and shifting photons insecond set of photons in the plurality of second channels by zero ormore channels, based on second configuration information provided to thesecond switch.

In some embodiments, a device includes a plurality of photon sourcescoupled to a plurality of output terminals. The plurality of photonsources are coupled together, by a first switch layer, into m groups ofn photon sources per group. The first switch layer includes m n-by-nswitches, each of the m n-by-n switches is coupled to the n photonsources per group. The device further includes a second switch layercoupled to output terminals of the first switch layer, and a pluralityof second layer n-by-n switches. At least two output terminals from tworespective photon sources residing within a first photon source groupand a second photon source group are coupled directly to respectiveoutput terminals without being coupled to any intervening second switchfrom the second switch layer. Each switch from the second switch layercan have less than m inputs and m outputs.

In some embodiments, the device further includes a first outermostsecond layer switch that is coupled to at least the first photon sourcegroup and is a 2-by-2 switch. The first outermost second layer switch isnot directly coupled to the second photon source group. The devicefurther includes a second outermost second layer switch that is coupledto at least the second photon group and is a 2-by-2 switch. The secondoutermost second layer switch is not directly coupled to the firstphoton source group.

In some embodiments, the plurality of switches from the second switchlayer are l-by-l switches and wherein l is less than m.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various described embodiments,reference should be made to the Detailed Description below, inconjunction with the following drawings in which like reference numeralsrefer to corresponding parts throughout the figures.

FIGS. 1A-1B are schematic diagrams illustrating a device formultiplexing photons (e.g., from single-photon sources) in accordancewith some embodiments.

FIGS. 2A-2F are schematic diagrams illustrating an example operation ofa device for multiplexing photons in accordance with some embodiments.

FIG. 3A is a schematic diagram illustrating a two-channel photon switchin accordance with some embodiments.

FIG. 3B is a schematic diagram illustrating a four-channel photon switchin accordance with some embodiments.

FIG. 4 is a schematic diagram illustrating an interferometer thatincludes photonic channels in accordance with some embodiments.

FIG. 5 is a schematic diagram illustrating a photon-delay component inaccordance with some embodiments.

FIG. 6 is a flowchart illustrating a method for multiplexing photons inaccordance with some embodiments.

FIGS. 7A-7B are schematic diagrams illustrating a device formultiplexing photons in accordance with some embodiments.

FIGS. 8A-8B are schematic diagrams illustrating a device formultiplexing photons in accordance with some embodiments.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings. In the following detaileddescription, numerous specific details are set forth in order to providea thorough understanding of the various described embodiments. However,it will be apparent to one of ordinary skill in the art that the variousdescribed embodiments may be practiced without these specific details.In other instances, well-known methods, procedures, components,circuits, and networks have not been described in detail so as not tounnecessarily obscure aspects of the embodiments.

As used herein, a single-photon source refers to a light source that isconfigured to emit a single-photon at a respective time. As explainedherein, a single-photon source need not emit a single-photon every timethere is an attempt to generate a single-photon (e.g., the success ratemay be less than 100%). In some embodiments, a single-photon sourcegenerates more than one photon (e.g., two photons) but includes amechanism to emit only a single-photon of the generated photons (e.g., asingle-photon source concurrently generates two photons and detects afirst photon of the two photons as a confirmation of the photongeneration and emits a second photon of the two photons, therebyemitting only a single-photon). As used herein, generating a photonincludes converting an energy (e.g., electric, magnetic, mechanical,thermal, and/or optical) into light. For example, a photon may begenerated from an electro-optical element (e.g., a semiconductor device,such as a light emitting diode and/or a chemical element, such as anorganic compound) or from an optical conversion process (e.g., four-wavemixing, spontaneous parametric down conversion, etc.).

As used herein, generating a single-photon refers to outputting a singlephoton in a predefined channel. In some embodiments, generating asingle-photon includes producing more than one photon (e.g., producingtwo or more photons during an intermediate operation) and directing onlyone photon in the predefined channel. In some embodiments, the rest ofthe produced photons, other than the only one photon directed in thepredefined channel, are destroyed (e.g., by detecting such photons),discarded, or transmitted to other channels. For example, in someembodiments, generating a single-photon refers to generating a heraldedsingle-photon, as explained below, and its corresponding heraldingphoton.

Although the photon-multiplexing principles described herein aredescribed with reference to single-photons, it should be understood thatthese photon-multiplexing principles are generally applicable to opticalmodes with one or more photons.

FIGS. 1A-1B are schematic diagrams illustrating devices for multiplexingphotons (e.g., schematic diagrams of embodiments of a photonicmultiplexing network) in accordance with some embodiments. In someembodiments, the photons are produced by single-photon sources (e.g.,probabilistic single-photon sources). FIGS. 1A-1B are described togetherand are analogous with the exception of the differences noted below.

FIG. 1A illustrates a device 100. In some embodiments, device 100 is aphotonic device. In some embodiments, device 100 is a hybridelectronic/photonic device (e.g., device 100 includes both electronicand photonic components).

Some embodiments of device 100 can be a multiplexing device, alsoreferred to herein as a photon multiplexer, that increases the apparentefficiency of single-photon sources 112 by using a plurality of photonsource 112 and multiplexing the outputs of the plurality of photonsources 112 (e.g., grouping a plurality of photon sources 112 into setsof photon sources 110 and multiplexing the plurality of photon sources112 in the set of photon sources 110 so that the set of photon sources110 has the characteristics of a single-photon source with a higherefficiency than an individual photon source 112). This is most easilyunderstood by looking at the embodiment of device 100 illustrated inFIGS. 1A-1B. As an example, assuming that each photon source 112 has a9% probability of producing a single-photon, there is a 31.5%probability that at least one of photon sources 112-a through 112-dproduces a single-photon. Thus, there is a 31.5% probability that firstset of photon sources 110-a (which includes photon sources 112-a through112-d) can be used to produce a single-photon, which is shifted topredetermined first channel 104-a and output onto second channel 108-a.If additional unwanted photons (e.g., extra photons) are produced byphoton sources 112-a through 112-d, the unwanted photons may bediscarded. In other examples, additional photons may not be discardedbut instead routed to one of the outputs in the device. In someembodiments, each photon source 112 is coupled to a source outputchannel of a plurality of source output channels through which photonsgenerated by the photons sources 112 travel to first switches 102, asdescribed below. For example, first channels 104 may serve as sourceoutput channels for the photon sources 112 (e.g., first channel 104-acoupled to photon source 112-a serves as a source output channel forphoton source 112-a).

Device 100 includes a plurality of first switches 102 (e.g., firstswitch 102-a, first switch 102-b, first switch 102-c, and first switch102-d). In some embodiments, the plurality of first switches 102comprises a first switching layer of device 100. Each first switch 102in the plurality of first switches includes a plurality of firstchannels 104 (e.g., first switch input channels). For example, eachfirst switch 102 includes a corresponding set of two or more firstchannels (e.g., first switch 102-a includes first channels 104-a through104-d; first switch 102-b includes first channels 104-e through 104-h;first switch 102-c includes first channels 104-i through 104-l; andfirst switch 102-d includes first channels 104-m through 104-p). In someembodiments, each first switch 102 is coupled to two or more firstswitch output channels. For example, second channels 108 may serve asfirst switch output channels (e.g., second channel 108-a coupled tofirst switch 102-a serves as a first switch output channel for firstswitch 102-a). In some embodiments, each first switch 102 includes thetwo or more first switch output channels (e.g., a portion of secondchannel 108-a serves as a first switch output channel for first switch102-a and a remaining portion of second channel 108-a serves as a secondswitch input channel for second switch 106-a). In some embodiments, thefirst switch output channels are a continuation of the first switchinput channels. In some embodiments, the plurality of first switch inputchannels of each first switch 102 are respectively coupled to a subsetof the plurality of source output channels from a subset of theplurality of photon sources. As explained with reference to FIGS. 2A-2F,each first switch 102 is configured to shift photons in the plurality offirst channels 104 by zero or more channels, based on configurationinformation provided to the first switch.

In some embodiments, device 100 further includes a plurality of secondswitches 106 (e.g., second switches 106-a through 106-d). In someembodiments, the plurality of second switches comprises a secondswitching layer of the device 100. The second switches 106 include aplurality of second channels 108 (e.g., each second switch 106 includesa plurality of second channels 108). For example, each second switch 106includes a corresponding set of two or more second channels (e.g.,second switch 106-a includes second channels 108-a, 108-e, 108-i, and108-m; second switch 106-b includes second channels 108-b, 108-f, 108-j,and 108-n; second switch 106-c includes second channels 108-c, 108-g,108-k, and 108-o; and second switch 106-d includes second channels108-d, 108-h, 108-l, and 108-p). In some embodiments, the secondchannels are second switch input channels. In some embodiments, eachsecond switch 106 is coupled to two or more second switch outputchannels (e.g., second switch 106-a is coupled to second switch outputchannels 118-a, 118-e, 118-i, and 118-m). In some embodiments, eachsecond switch 106 includes the two or more second switch outputchannels. In some embodiments, the second switch output channels are acontinuation of the second switch input channels.

For each second switch 106, a second channel 108 within the secondswitch 106 is coupled with a respective first channel 104 from adistinct first switch 102 of the plurality of first switches 102. Insome embodiments, each second channel 108 within each second switch iscoupled with a respective first channel 104 (FIG. 1A). In someembodiments, one second channel 108 within each second switch is coupledwith a respective first channel 104 (FIG. 1B). In some embodiments, eachsecond switch 106 is coupled, by the corresponding set of secondchannels, to outputs of two or more first switches 102. For example,second switch 106-a is coupled to the outputs of four first switches 102by its corresponding set of second channels 108 (e.g., second switch106-a is coupled to: first switch 102-a by second channel 108-a; tofirst switch 102-b by second channel 108-e; to first switch 102-c bysecond channel 108-i; and to first switch 102-d by second channel108-m). In some embodiments, two respective second switch input channelsof each second switch 106 are coupled to two different first switchoutput channels from two different first switches 102.

As described further with reference to FIGS. 2A-2F, in some embodiments,each first switch 102 is configured to output, in accordance with adetermination that one or more photon-availability criteria are met, asingle-photon to a predetermined first channel 104 within the firstswitch 102. In some embodiments, the photon-availability criteria aremet when it is possible to output a single-photon to the predeterminedfirst channel 104 within first switch 102 (e.g., when at least onephoton source 112 in the corresponding set of photon sources 110 hasproduced a single-photon). In some embodiments, other single-photonsproduced by the corresponding set of photon sources 110 can bediscarded. Thus, in some embodiments, there is no need to couple theother first channels 104 (e.g., the first channels 104 that are notselected to receive a single-photon as an output from the first switch102) with the second channels 108 in the second switches 106. Device 100in FIG. 1A is a schematic diagram illustrating an embodiment in whichthe other first channels 104 are coupled with second channels 108. Tothat end, in some embodiments, first switches 102 are N×N switches(where N is the number of first channels 104). An N×N switch is a switchthat couples N input channels to N output channels, e.g., as implementedby a generalized Mach Zehnder interferometer (MZI). Device 120 in FIG.1B is a schematic diagram illustrating an embodiment in which the otherfirst channels 104 are not coupled with second channels 108. To thatend, in some embodiments, first switches 102 are N×1 switches. An N×1switch is a switch that couples N input channels to a singlepredetermined channel. In some embodiments, second switches 106 are N×Nswitches. In some embodiments, second switches 106 are N×1 switches.

In some embodiments, a second switch 106 is coupled with each firstswitch 102 by a distinct respective second channel 108 (e.g., for eachsecond switch 106, at least one second channel 108 within the secondswitch 106 is coupled to each first switch 102). For example, secondswitch 106-a includes second channels 108-a, 108-e, 108-i, 108-m. Secondchannel 108-a is coupled with (e.g., connected to) first channel 104-awithin first switch 102-a. Second channel 108-e is coupled with (e.g.,connected to) first channel 104-e within first switch 102-b. Secondchannel 108-i is coupled with (e.g., connected to) first channel 104-iwithin first switch 102-c. Second channel 108-m is coupled with (e.g.,connected to) first channel 104-m within first switch 102-d.

As another example, second switch 106-b includes second channels 108-b,108-f, 108-j, 108-n. Second channel 108-b is coupled with (e.g.,connected to) first channel 104-b within first switch 102-a. Secondchannel 108-f is coupled with (e.g., connected to) first channel 104-fwithin first switch 102-b. Second channel 108-g is coupled with (e.g.,connected to) first channel 104-g within first switch 102-c. Secondchannel 108-n is coupled with (e.g., connected to) first channel 104-nwithin first switch 102-d.

As another example, second switch 106-c includes second channels 108-c,108-g, 108-k, 108-o. Second channel 108-c is coupled with (e.g.,connected to) first channel 104-c within first switch 102-a. Secondchannel 108-g is coupled with (e.g., connected to) first channel 104-gwithin first switch 102-b. Second channel 108-k is coupled with (e.g.,connected to) first channel 104-k within first switch 102-c. Secondchannel 108-o is coupled with (e.g., connected to) first channel 104-owithin first switch 102-d.

As another example, second switch 106-d includes second channels 108-d,108-h, 108-l, 108-p. Second channel 108-d is coupled with (e.g.,connected to) first channel 104-d within first switch 102-a. Secondchannel 108-h is coupled with (e.g., connected to) first channel 104-hwithin first switch 102-b. Second channel 108-l is coupled with (e.g.,connected to) first channel 104-l within first switch 102-c. Secondchannel 108-p is coupled with (e.g., connected to) first channel 104-pwithin first switch 102-d.

In some embodiments, first channels 104 are photonic channels, e.g.,such as integrated photonics channels. In some embodiments, secondchannels 108 are photonic channels. For example, a photonic channel is aphotonic channel (e.g., a waveguide) fabricated on a chip (e.g., usingoptical or e-beam lithographic processes). For example, a photonicchannel includes two materials (e.g., one of which may comprise asubstrate of the chip, such as a semiconductor such as Si, asemiconductor oxide such as SiO₂) that have a large differential indexof refraction (e.g., a large difference in the index of refraction ofthe first material and the index of refraction of the second material).In some embodiments, each channel has a width on the order of tens ofnanometers (e.g., 10 nm, 50 nm). In some embodiments, each channel has awidth on the order of microns (e.g., 1 micron, 10 microns).

In some embodiments, a respective first channel 104 and a respectivesecond channel 108 (e.g., that is coupled with the respective firstchannel 104) are portions of a larger channel. For example, firstchannel 104-a and second channel 108-a may be portions of a singlephotonic channel fabricated on a chip.

As explained with reference to FIGS. 2A-2F, each second switch 106 isconfigured to shift photons in the plurality of second channels 108 byzero or more channels, based on configuration information provided tothe second switch 106.

In some embodiments, each first switch 102 of the plurality of firstswitches 102 corresponds to a distinct set of photon sources 110. Insome embodiments, the sets of photon sources 110 are included in device100. In some embodiments, the sets of photon sources 110 are external todevice 100 (e.g., coupled to device 100 through an interface with firstchannels 104). In some embodiments, each set of photon sources 110includes a plurality of photon sources 112 (e.g., 2, 3, 4, 8, or 16photon sources 112). For example, each set of photon sources 110, FIGS.1A-1B, includes four photon sources 112. First switch 102-a correspondsto first set of photon sources 110-a, which includes photon sources112-a through 112-d; first switch 102-b corresponds to second set ofphoton sources 110-b, which includes photon sources 112-e through 112-h;first switch 102-c corresponds to third set of photon sources 110-c,which includes photon sources 112-i through 112-l; and first switch102-d corresponds to fourth set of photon sources 110-d, which includesphoton sources 112-m through 112-p.

In some embodiments, device 100 includes one first switch 102 for eachset of photon sources 110. In some embodiments, each respective firstswitch 102 is connected to a corresponding set of photon sources 110 bythe first channels 104 of the respective first switch 102. In someembodiments, there is intervening electronic circuitry or photoniccomponentry between each set of photon sources 110 and the correspondingfirst switch 102. In some embodiments, each set of photon sources 110 iscoupled with exactly one first switch 102 and each first switch 102 iscoupled with exactly one set of photon sources 110. In some embodiments,each set of photon sources 110 includes a number (e.g., count) of photonsources 112; each first switch 102 includes the same number of firstchannels 104; each first channel 104 is coupled with exactly one photonsource 112; and each photon source 112 is coupled with exactly one firstchannel 104.

In some embodiments, photon sources 112 are probabilistic photonsources. For example, photon sources 112 have a photon-numberdistribution (e.g., a distribution of numbers of photons produced perattempt) with a non-zero variance. In some embodiments, a respectivephoton source 112 is most likely to, on a respective attempt, producezero photons (e.g., there is a 90% probability of producing zero photonsper attempt to produce a single-photon). The second most likely resultfor an attempt is production of a single-photon (e.g., there is a 9%probability of producing a single-photon per attempt to produce asingle-photon). The third most likely result for an attempt isproduction of two photons (e.g., there is a 1% probability of producingtwo photons per attempt to produce a single-photon). In somecircumstances, there is less than 1% probability of producing more thantwo photons.

In some embodiments, the single-photons produced by photon sources 112are heralded single-photons. Heralded single-photons can be produced ina variety of ways. For example, in some embodiments, the photon sources112 include a laser or any other light source, e.g., LEDs, and the like.The laser produces a laser beam, referred to as a pump or a pump beam(which includes pump photons). In some embodiments, the laser producesmany photons either continuously or in bursts (e.g., pulses). A photonpair is created by converting one pump photon into a pair of photonshaving lower energy than the pump photon (e.g., using a material havinga second-order non-linear coefficient). One of the photons is then usedto herald the presence of the other one.

Alternatively, in some embodiments, two photons from a pump areconverted into a pair of photons. One photon of the pair of photons hasa lower energy than a respective pump photon. The other photon of thepair of photons has higher energy than the respective pump photon. Oneof photons (e.g., either the higher-energy photon or the lower-energyphoton) is then used to herald the presence of the other photon.

Thus, production of a heralded photon produces the heralded photon aswell as a heralding photon. In some circumstances, one photon of thepair of photons is outputted (e.g., onto a first channel 104) while theother is used to “herald” the arrival of the outputted photon. Thus, theoutputted photon is sometimes referred to herein as a “heralded” photonand the other photon of the pair of photons is referred to as a“heralding” photon. Typically, the heralding photon is destroyed in theprocess.

In some embodiments, as explained in greater detail below with referenceto FIGS. 3A-3B (and not shown in FIGS. 1A-1B), device 100 includes, foreach photon source 112, circuitry to determine whether the photon source112 has emitted a photon (e.g., by detecting the heralding photon). Insome embodiments, as described with reference to FIGS. 3A-3B, device 100uses the detected heralding photon as configuration information toconfigure the corresponding first switch 102 to select the respectivephoton source 112 as having produced a photon (e.g., the heraldedphoton). In some embodiments, the configuration information (e.g., theheralding photon) is used to configure a set of phase shifters withinthe corresponding first switch 102. The set of phase shifters is used toselect, for output, one of the photon sources 112 in the correspondingset of photon sources 110.

In some embodiments, the plurality of second switches 106 is coupledwith a plurality of sets 114 of device output terminals 116 (e.g., haveoutputs that are coupled with a plurality of device output terminals116). In some embodiments, the plurality of first switches 102 and theplurality of second switches 106 are configured to shift n photonsrespectively generated by n photon sources that are a subset of theplurality of photon sources 112 to a predetermined subset of theplurality of device output terminals 116 (e.g., a respective set 114 ofdevice output terminals 116) based on configuration information thatindicates the subset of photon sources 112 that generated the n photons.

In some embodiments, a respective set 114 of device output terminals 116includes a plurality of device output terminals 116 (e.g., 2, 3, 4, 8,or 16 device output terminals). For example, each set 114 of deviceoutput terminals 116, FIGS. 1A-1B, includes four device output terminals116. For example, first set 114-a of device output terminals 116includes device output terminals 116-a through 116-d; second set 114-bof device output terminals 116 includes device output terminals 116-ethrough 116-h; third set 114-c of device output terminals 116 includesdevice output terminals 116-i through 116-l; and fourth set 114-d ofdevice output terminals 116 includes device output terminals 116-mthrough 116-p.

In some embodiments, for each second switch 106, a respective secondchannel 108 is coupled to a respective device output terminal 116 of adistinct set 114 of device output terminals 116. For example, eachsecond switch 106 includes four second channels 108, each coupled to adevice output terminal 116 from a different set 114 of device outputterminals 116. For example, second switch 106-a includes: second channel108-a coupled with device output terminal 116-a (part of the first set114-a of device output terminals 116); second channel 108-e coupled withdevice output terminal 116-e (part of the second set 114-b of deviceoutput terminals 116); second channel 108-i coupled with device outputterminal 116-i (part of the third set 114-c of device output terminals116); and second channel 108-m coupled with device output terminal 116-m(part of the fourth set 114-d of device output terminals 116).

In some embodiments, each second switch 106 includes exactly one secondchannel 108 coupled with each set 114 of device output terminals 116(e.g., exactly one second channel 108 coupled with a respective deviceoutput terminal 116 within each set 114 of device output terminals 116).In some embodiments, each second switch 106 includes no more than onesecond channel 108 coupled with each set 114 of device output terminals116 (e.g., each second switch 106 includes respective second channels108 coupled with some, but not all, of the second switches 106). In someembodiments, each second switch 106 includes at least one second channel108 coupled with each set 114 of device output terminals 116.

In some embodiments, each set 114 of device output terminals 116includes exactly one device output terminal 116 coupled with each secondswitch 106 (e.g., coupled with a respective second channel 108 withineach second switch 106). In some embodiments, each set 114 of deviceoutput terminals 116 includes no more than one device output terminal116 coupled with each second switch 106 (e.g., each set 114 of deviceoutput terminals 116 includes respective device output terminals 116coupled with some, but not all, of the second switches 106). In someembodiments, each set 114 of device output terminals 116 includes atleast one device output terminal 116 coupled with each second switch106.

In some embodiments, device output terminals 116 are photonic channels.As noted above, in some embodiments, second channels 108 are photonicchannels. In some embodiments, a respective second channel 108 and arespective device output terminal 116 (e.g., that is coupled with therespective second channel 108) are portions of a larger channel (e.g.,that includes a respective first channel 104, coupled with therespective channel 108 on the other side). For example, second channel108-a and device output terminal 116-a may be portions of a singlephotonic channel that has been fabricated on a chip.

Thus, in some embodiments, device 100 is a two-layer photonicmultiplexer, comprising a first switching layer (e.g., first switches102) and a second switching layer (e.g., second switches 106). In someembodiments, the first switching layer produces a set of single-photonoutputs (e.g., first channels 104-a, 104-f, 104-k, and 104-p) that havethe characteristics of high-efficiency single-photon sources. In someembodiments, the second switching layer selects a set 114 of deviceoutput terminals 116 for outputting the photons from the set ofhigh-efficiency single-photon outputs (e.g., the second switching layerselects the set 114-a of device output terminals 116 for output, or theset 114-b of device output terminals 116, or the set 114-c of deviceoutput terminals 116, or the set 114-d of device output terminals 116).In some embodiments, a device is provided that includes only one of thetwo switching layers (e.g., the present disclosure provides the firstswitching layer without requiring the second switching layer as well asthe second switching layer without requiring the first switching layer).

The following is an alternate description of device 100 in accordancewith some embodiments.

Device 100 includes switch 102-a (e.g., a first switch) coupled with(e.g., includes) channel 104-a (e.g., a first channel) and channel 104-b(e.g., another first channel). Switch 102-a is configured to shiftphotons by zero or more channels based on first configurationinformation provided to the switch 102-a (e.g., by a first phaseselector analogous to phase selector 316-b, as shown in FIG. 3B below),including (i) maintaining a photon in channel 104-a and maintaining aphoton in channel 104-b when the first configuration informationindicates shifting by zero channels and (ii) shifting the photon in thechannel 104-a to the channel 104-b and shifting the photon in thechannel 104-b to a channel that is distinct from second channel 104-bwhen the first configuration information indicates shifting by onechannel.

Device 100 further includes switch 102-b (e.g., another first switch)coupled with channel 104-e (another first channel) and channel 104-f(e.g., another first channel). Switch 102-b is configured to shiftphotons by zero or more channels based on second configurationinformation provided to switch 102-b (e.g., by a second phase selectoranalogous to phase selector 316-b, FIG. 3B), including (i) maintaining aphoton in channel 104-e and maintaining a photon in channel 104-f whenthe second configuration information indicates shifting by zero channelsand (ii) shifting the photon in channel 104-e to channel 104-f andshifting the photon in channel 104-f to a channel that is distinct fromchannel 104-f when the second configuration information indicatesshifting by one channel.

Device 100 further includes switch 106-a (e.g., a second switch) coupledwith channel 104-a (via channel 108-a) and channel 104-e (via channel108-e). Switch 106-a is configured to shift photons by zero or morechannels based on third configuration information provided to switch106-a, including (i) maintaining a photon in channel 104-a andmaintaining a photon in channel 104-e when the third configurationinformation indicates shifting by zero channels and (ii) shifting thephoton in channel 104-a to channel 104-e and shifting the photon inchannel 104-e to a channel that is distinct from channel 104-e when thethird configuration information indicates shifting by one channel. Insome embodiments, the third configuration information is provided by athird phase selector analogous to phase selector 316-b, FIG. 3B, exceptthat the phases are selected in accordance with desired set of deviceoutput terminals rather than detection of photons.

Device 100 further includes switch 106-b (e.g., another second switch)coupled with channel 104-b (via channel 108-b) and channel 104-f (viachannel 108-f). Switch 106-b is configured to shift photons by zero ormore channels based on fourth configuration information provided toswitch 106-b, including (i) maintaining a photon in channel 104-b andmaintaining a photon in channel 104-f when the fourth configurationinformation indicates shifting by zero channels and (ii) shifting thephoton in channel 104-b to channel 104-f and shifting the photon inchannel 104-f to a channel that is distinct from channel 104-f when thefourth configuration information indicates shifting by one channel. Insome embodiments, the fourth configuration information is provided by afourth phase selector analogous to phase selector 316-b, FIG. 3B, exceptthat the phases are selected in accordance with desired set of deviceoutput terminals rather than detection of photons.

In some embodiments, switch 102-a is configured to shift the photon inchannel 104-b to channel 104-a when the first configuration informationindicates shifting by one channel and switch 102-b is configured toshift the photon in channel 104-f to channel 104-e when the secondconfiguration information indicates shifting by one channel.

In some embodiments, switch 102-a is coupled with (e.g., includes)channel 104-c (e.g., another first channel) in addition to channel 104-aand channel 104-b. Switch 102-a is configured to (iii) shift the photonin channel 104-a to channel 104-c, shift the photon in channel 104-b tochannel 104-a, and shift the photon in channel 104-c to channel 104-bwhen the first configuration information indicates shifting by twochannels.

In some embodiments, the switch 102-b is coupled with channel 104-g(e.g., another first channel) in addition to channel 104-e and channel104-f. Switch 102-b is configured to (iii) shift the photon in channel104-e to channel 104-g, shift the photon in channel 104-f to channel104-e, and shift the photon in channel 104-g to channel 104-f when thesecond configuration information indicates shifting by two channels.

In some embodiments, switch 106-a is configured to shift the photon inchannel 104-e to channel 104-a when the third configuration informationindicates shifting by one channel and switch 106-b is configured toshift the photon in channel 104-f to channel 104-b when the fourthconfiguration information indicates shifting by one channel.

In some embodiments, switch 102-a is configured to (i) maintain a photonin channel 104-a, maintain a photon in channel 104-b, and maintain aphoton in channel 104-c when the first configuration informationindicates shifting by zero channels and (ii) shift the photon in channel104-a to channel 104-b, shift the photon in channel 104-b to channel104-c, and shift the photon in channel 104-c to a channel that isdistinct from channel 104-c when the first configuration informationindicates shifting by one channel.

In some embodiments, switch 102-b is coupled with (e.g., includes)channel 104-g in addition to channel 104-e and channel 104-f. Switch102-b is configured to (i) maintain a photon in channel 104-e, maintaina photon in channel 104-f, and maintain a photon in channel 104-g whenthe second configuration information indicates shifting by zero channelsand (ii) shift the photon in channel 104-e to channel 104-f, shift thephoton in channel 104-f to channel 104-g, and shift the photon inchannel 104-g to a channel that is distinct from channel 104-g when thesecond configuration information indicates shifting by one channel.

In some embodiments, device 100 further includes switch 106-c (e.g.,another second switch) that is coupled with channel 104-c (via channel108-c) and channel 104-g (via channel 108-g). Switch 106-c is configuredto shift photons by zero or more channels based on fifth configurationinformation provided to switch 106-c, including (i) maintaining a photonin channel 104-c and maintaining a photon in channel 104-g when thefifth configuration information indicates shifting by zero channels and(ii) shifting the photon in channel 104-c to channel 104-g and shiftingthe photon in channel 104-g to a channel that is distinct from channel104-g when the fifth configuration information indicates shifting by onechannel. In some embodiments, the fifth configuration information isprovided by a fifth phase selector analogous to phase selector 316-b,FIG. 3B, except that the phases are selected in accordance with desiredset of device output terminals rather than detection of photons.

In some embodiments, device 100 includes switch 102-c (e.g., a firstswitch) that is coupled with (e.g., includes) channel 104-i, channel104-j, and channel 104-k (e.g., distinct additional first channels).Switch 102-c is configured to shift photons by zero or more channelsbased on sixth configuration information provided to switch 102-c (e.g.,by a sixth phase selector analogous to phase selector 316-b, FIG. 3B),including (i) maintaining a photon in channel 104-i, maintaining aphoton in channel 104-j, and maintaining a photon in channel 104-k whenthe sixth configuration information indicates shifting by zero channelsand (ii) shifting the photon in channel 104-i to channel 104-j, shiftingthe photon in channel 104-j to channel 104-k, and shifting the photon inchannel 104-k to a channel that is distinct from channel 104-k when thesixth configuration information indicates shifting by one channel. Insome embodiments, the sixth configuration information is provided by asixth phase selector analogous to phase selector 316-b, FIG. 3B, exceptthat the phases are selected in accordance with desired set of deviceoutput terminals rather than detection of photons.

In some embodiments, switch 106-a is coupled with channel 104-i (viachannel 108-i) in addition to channel 104-a and channel 104-e. Switch106-a is configured to (i) maintain a photon in channel 104-a, maintaina photon in channel 104-e, and maintain a photon in channel 104-i whenthe third configuration information indicates shifting by zero channelsand (ii) shift the photon in channel 104-a to channel 104-e, shift thephoton in channel 104-e to channel 104-i, and shift the photon inchannel 104-i to a channel that is distinct from channel 104-i when thethird configuration information indicates shifting by one channel.

In some embodiments, switch 106-b is coupled with channel 104-j (viachannel 108-j) in addition to channel 104-b and channel 104-f Switch106-b is configured to (i) maintain a photon in channel 104-b, maintaina photon in channel 104-f, and maintain a photon in channel 104-j whenthe fourth configuration information indicates shifting by zero channelsand (ii) shift the photon in channel 104-b to channel 104-f, shift thephoton in channel 104-f to channel 104-j, and shift the photon inchannel 104-j to a channel that is distinct from channel 104-j when thefourth configuration information indicates shifting by one channel.

In some embodiments, switch 106-c is coupled with channel 104-k (viachannel 108-k) in addition to channel 104-c and channel 104-g. Switch106-c is configured to (i) maintain a photon in channel 104-c, maintaina photon in channel 104-g, and maintain a photon in channel 104-k whenthe fifth configuration information indicates shifting by zero channelsand (ii) shift the photon in channel 104-c to channel 104-g, shift thephoton in channel 104-g to channel 104-k, and shift the photon inchannel 104-k to a channel that is distinct from channel 104-k when thefifth configuration information indicates shifting by one channel.

In some embodiments, switch 106-a is configured to (iii) shift thephoton in channel 104-a to channel 104-i, shift the photon in channel104-e to channel 104-a, and shift the photon in channel 104-i to channel104-e when the third configuration information indicates shifting by twochannels.

In some embodiments, switch 106-b is configured to (iii) shift thephoton in channel 104-b to channel 104-j, shift the photon in channel104-f to channel 104-b, and shift the photon in channel 104-j to channel104-f when the fourth configuration information indicates shifting bytwo channels.

In some embodiments, switch 106-c is configured to (iii) shift thephoton in channel 104-c to channel 104-k, shift the photon in channel104-g to channel 104-c, and shift the photon in channel 104-k to channel104-g when the fifth configuration information indicates shifting by twochannels.

In some embodiments, switch 102-a is coupled with (e.g., includes)channel 104-d (e.g., another first channel) in addition to channel104-a, channel 104-9 b, and channel 104-c. Switch 102-a is configured to(i) maintain a photon in channel 104-a, maintain a photon in channel104-b, maintain a photon in channel 104-c, and maintain a photon inchannel 104-d when the first configuration information indicatesshifting by zero channels and (ii) shift the photon in channel 104-a tochannel 104-b, shift the photon in channel 104-b to channel 104-c, shiftthe photon in channel 104-c to channel 104-d, and shift the photon inchannel 104-d to a channel that is distinct from channel 104-d when thefirst configuration information indicates shifting by one channel.

In some embodiments, switch 102-b is coupled with (e.g., includes)channel 104-h (e.g., another first channel) in addition to channel104-e, channel 104-f, and channel 104-g. Switch 102-b is configured to(i) maintain a photon in channel 104-e, maintain a photon in channel104-f, maintain a photon in channel 104-g, and maintain a photon inchannel 104-h when the second configuration information indicatesshifting by zero channels and (ii) shift the photon in channel 104-e tochannel 104-f, shift the photon in channel 104-f to channel 104-g, shiftthe photon in channel 104-g to channel 104-h, and shift the photon inchannel 104-h to a channel that is distinct from channel 104-h when thesecond configuration information indicates shifting by one channel.

In some embodiments, switch 102-c is coupled with (e.g., includes)channel 104-l (e.g., another first channel) in addition to channel104-i, channel 104-j, and channel 104-k. Switch 102-c is configured to(i) maintain a photon in channel 104-i, maintain a photon in channel104-j, maintain a photon in channel 104-k, and maintain a photon inchannel 104-l when the sixth configuration information indicatesshifting by zero channels and (ii) shift the photon in channel 104-i tochannel 104-j, shift the photon in channel 104-j to channel 104-k, shiftthe photon in channel 104-k to channel 104-l, and shift the photon inchannel 104-l to a channel that is distinct from channel 104-l when thesixth configuration information indicates shifting by one channel.

In some embodiments, device 100 further includes switch 102-d (e.g.,another first switch) that is coupled with (e.g., includes) channel104-m, channel 104-n, channel 104-o, and channel 104-p (e.g., distinctadditional channels). Switch 102-d is configured to shift photons byzero or more channels based on seventh configuration informationprovided to switch 102-d, including (i) maintaining a photon in channel104-m, maintaining a photon in channel 104-n, maintaining a photon inchannel 104-o, and maintaining a photon in channel 104-p when theseventh configuration information indicates shifting by zero channelsand (ii) shifting the photon in channel 104-m to channel 104-n, shiftingthe photon in channel 104-n to channel 104-o, shifting the photon inchannel 104-o to channel 104-p, and shifting the photon in channel 104-pto a channel that is distinct from channel 104-p when the seventhconfiguration information indicates shifting by one channel. In someembodiments, the seventh configuration information is provided by aseventh phase selector analogous to phase selector 316-b, FIG. 3B,except that the phases are selected in accordance with desired set ofdevice output terminals rather than detection of photons.

In some embodiments, switch 106-a is coupled with channel 104-m (viachannel 108-m) in addition to channel 104-a, channel 104-e, and channel104-i. Switch 106-a is configured to (i) maintain a photon in channel104-a, maintain a photon in channel 104-e, maintain a photon in channel104-i, and maintain a photon in channel 104-m when the thirdconfiguration information indicates shifting by zero channels and (ii)shift the photon in channel 104-a to channel 104-e, shift the photon inchannel 104-e to channel 104-i, shift the photon in channel 104-i tochannel 104-m, and shift the photon in channel 104-m to a channel thatis distinct from channel 104-m when the third configuration informationindicates shifting by one channel.

In some embodiments, switch 106-b is coupled with channel 104-n (viachannel 108-n) in addition to channel 104-b, channel 104-f, and channel104-j. Switch 106-b is configured to (i) maintain a photon in channel104-b, maintain a photon in channel 104-f, maintain a photon in channel104-j, and maintain a photon in channel 104-n when the fourthconfiguration information indicates shifting by zero channels and (ii)shift the photon in channel 104-b to channel 104-f, shift the photon inchannel 104-f to channel 104-j, shift the photon in channel 104-j tochannel 104-n, and shift the photon in channel 104-n to a channel thatis distinct from channel 104-n when the fourth configuration informationindicates shifting by one channel.

In some embodiments, switch 106-c is coupled with channel 104-o (viachannel 108-o) in addition to channel 104-c, channel 104-g, and channel104-k. Switch 106-c is configured to (i) maintain a photon in channel104-c, maintain a photon in channel 104-g, maintain a photon in channel104-k, and maintain a photon in channel 104-o when the fifthconfiguration information indicates shifting by zero channels and (ii)shift the photon in channel 104-c to channel 104-g, shift the photon inchannel 104-g to channel 104-k, shift the photon in channel 104-k tochannel 104-o, and shift the photon in channel 104-o to a channel thatis distinct from channel 104-o when the fifth configuration informationindicates shifting by one channel.

In some embodiments, device 100 further includes switch 106-d (e.g.,another second switch) that is coupled with channel 104-d (via channel108-d), channel 104-h (via channel 108-h), channel 104-l (via channel108-l), and channel 104-p (via channel 108-p). Switch 106-d isconfigured to shift photons by zero or more channels based on eighthconfiguration information provided to switch 106-d (e.g., by an eighthphase selector analogous to phase selector 316-b, FIG. 3B), including(i) maintaining a photon in channel 104-d, maintaining a photon inchannel 104-h, maintaining a photon in channel 104-l, and maintaining aphoton in channel 104-p when the eighth configuration informationindicates shifting by zero channels and (ii) shifting the photon inchannel 104-d to channel 104-h, shifting the photon in channel 104-h tochannel 104-l, shifting the photon in channel 104-l to channel 104-p,and shifting the photon in channel 104-p to a channel that is distinctfrom channel 104-p when the eighth configuration information indicatesshifting by one channel.

FIGS. 2A-2F are schematic diagrams illustrating an example of theoperation of device 100 (e.g., a device for multiplexing photons) inaccordance with some embodiments.

FIG. 2A shows device 100 at a first time, immediately following anattempt by photon sources 112 to produce a photon (e.g., produce asingle-photon). In some embodiments, each photon source 112 hasattempted to produce a photon. In some embodiments, each photon source112 has simultaneously attempted to produce a photon (e.g., without anintentionally-introduced delay between the various photon sources 112).For example, prior to the first time, an electrical control signal toproduce a photon was sent to each photon source 112 withoutintentionally introducing a delay in the arrival time of the signal atthe various photon sources 112. As referred to below, an attempt to havesome or all of photon sources 112 produce a photon is referred to as a“shot.”

The result of a shot is a set of zero or more photons, represented asblack balls in FIGS. 2A-2F, produced by photon sources 112. For example,at the first time, a first photon has been produced by photon source112-c of first set of photon sources 110-a; a second photon has beenproduced by photon source 112-e of the second set of photon sources110-b; a third photon has been produced by photon source 112-k of thethird set of photon sources 110-c; and a fourth photon has been producedby photon source 112-m of the fourth set of photon sources 110-d. Insome circumstances, a number (e.g., count) of photons has been producedby each photon source 112 according to a corresponding probability ofproducing the number of photons (e.g., a probability of producing zerophotons; a probability of producing a single-photon; a probability ofproducing two photons). In some embodiments, a photon source issuccessful when it produces a single-photon (e.g., exactly one photon).

It should be noted that the distribution of photons produced by photonsources 112 after a shot could look entirely different from that shownin FIG. 2A. Each attempt to produce a photon at each photon source 112is analogous to a roll of the dice, with outcomes weighted by theprobabilities for producing different numbers of photons. For example,all four photon sources 112 in the first set of photon sources 110-a maybe successful on a respective shot (e.g., in which case there would be aphoton in each of photon source 112-a, 112-b, 112-c, and 112-d, ratherthan just a photon in photon source 112-c). As another example, somephoton sources 112 may produce two photons (which, in some embodiments,is considered unsuccessful because of downstream circuitry and opticalcomponentry that are only configured to handle single-photons inselected output terminals 116). In some circumstances, for a respectiveshot, a set of photon sources 110 may have failed to produce anysingle-photons. For example, for a respective shot, there may be zerophotons produced by one or more of the sets of photon sources 110 (e.g.,none of the photon sources 112 in the first set of photon sources 110-aproduced a photon) or a combination of zero photons and two or morephotons in a set of photon sources.

In some embodiments, device 100 determines whether the shot has beensuccessful based on one or more shot-success criteria. In someembodiments, the one or more success criteria include a condition thatis met when at least one photon source 112 in each set of photon sources110 has produced a single-photon. In some embodiments, the one or moresuccess criteria include a condition that is met when at least onephoton source 112 in each set of photon sources 110 has produced atleast one photon. In some embodiments, the one or more success criteriainclude a condition that is met when at least one photon source 112 in apredefined number (e.g., threshold number) of sets of photon sources 110has produced a single-photon (e.g., at least three of the four sets ofphoton sources 110 has produced a single-photon). In some embodiments,the one or more success criteria include a condition that is met when atleast one photon source 112 in a predefined number (e.g., thresholdnumber) of set of photon sources 110 has produced at least one photon.

In some embodiments, when the shot-success criteria have not been met,the shot is discarded and device 100 performs a second attempt toproduce a set of photons that meet the shot-success criteria. In someembodiments, when the shot is discarded, device 100 does not route thephotons to the device output terminals 116. Alternatively, in someembodiments, when the shot is discarded, the set of photons areterminated (e.g., destroyed) by circuitry and/or optical componentryexternal to device 100 (e.g., after device 100 has routed the set ofphotons to the device output terminals 116).

Alternatively, in some embodiments, the shot is not discarded even whenthe shot-success criteria are not met. For example, in some embodiments,device 100 is used in quantum computing applications based on linearoptics. To that end, the set of photons output by device 100 aresubsequently entangled into an entangled state. In some embodiments, theentangled state is an error-correcting code state. Assume, momentarily,that the shot-success criteria are not met because none of the photonsources 112 in the set of photon sources 110-a produced a single-photonin the shot, but each other set of photon sources 110 has produced asingle-photon. In some circumstances, even if the desirederror-correcting code state cannot be produced without a single-photonfrom the set of photon sources 110-a, the error-correcting code state isrobust enough to handle the defect (e.g., only a certain fraction of theerror correcting code state needs to be correct). Thus, device 100 doesnot have to be perfect, but, instead, need only be good enough given therobustness of the downstream processing and error correction. As anotherexample, even when the result of a shot is not good enough given therobustness of the downstream processing and error correction, the shotis not discarded. Rather, the downstream processing is attempted andallowed to fail. Subsequently, the entire process is repeated (e.g.,performed again). Allowing the process to proceed even when a shot failsis, in some circumstances, simpler, faster, or more convenient thandetecting a failure in the middle of the computational process.

In some embodiments, each single-photon shown in FIG. 2A is a heraldedphoton of a pair of photons. A photon source 112 is considered to haveproduced a “single-photon” when it produces a single heralded photon.

FIG. 2B shows device 100 at a second time that is after the first time.At the second time, the photons produced by the photon sources 112 havetraveled down their respective first channels 104 to their respectivefirst switches 102. For example, the first photon produced by photonsource 112-c has traveled down first channel 104-c to first switch102-a; the second photon produced by photon source 112-e has traveleddown first channel 104-e to first switch 102-b; the third photonproduced by photon source 112-k has travelled down first channel 104-kto first switch 102-c; and the fourth photon produced by photon source112-m has travelled down first channel 104-m to first switch 102-d.

The first switches 102 are configured to (e.g., configurable to), whenthe corresponding distinct set of photon sources 110 has emitted one ormore photons, shift a photon to a predetermined channel of the pluralityof first channels 104 within the corresponding first switch. Theconfiguration of each first switch 102 is based on configurationinformation provided to the first switch (e.g., detection or lack ofdetection of a heralding photon from each of the photon sources 112 inthe set of photon sources 110 corresponding to the first switch 102).

In some embodiments, each respective first switch 102 has a set ofinputs that is cyclically permuted onto a set of outputs of therespective first switch 102. For example, in some embodiments, thephoton sources 112 are ordered within their corresponding first set ofphoton sources 110 (e.g., as defined by their connection to the inputsof the corresponding first switch 102). The corresponding first switch102 is configured to shift the photons at the inputs, in sequence (e.g.,maintaining the order), to the predetermined channel. For example, thepredetermined channel for the first switch 102-a may be first channel104-a. The order for first set of photon sources 110-a may be photonsource 112-a, followed by photon source 112-b, followed by photon source112-c, followed by photon source 112-d. When a single-photon is presentin first channel 104-a (e.g., before entering first switch 102-a), firstswitch 102-a is configured to shift the single-photon in first channel104-a by zero channels (e.g., maintain the single-photon in channel104-a). Any photons in the remaining first channels 104 within firstswitch 102-a are not shifted to first channel 104-a (e.g., they aremaintained in their original channels or shifted to channels other thanchannel 104-a). When a single-photon is not present in first channel104-a, but there is a single-photon in second channel 104-b, firstswitch 102-a is configured to shift the single-photon in first channel104-b by one channel to first channel 104-a. Any photons in theremaining first channels 104 within first switch 102-a are not shiftedto first channel 104-a (e.g., they are shifted by one channel in thesame direction or otherwise shifted to channels other than first channel104-a). When a single-photon is not present in first channel 104-a or104-b, but there is a single-photon in second channel 104-c, firstswitch 102-a is configured to shift the single-photon in first channel104-c by two channels to first channel 104-a. Any photons in theremaining first channels 104 within first switch 102-a are not shiftedto first channel 104-a (e.g., they are shifted by two channels in thesame direction or otherwise shifted to first channels other than firstchannel 104-a). When a single-photon is not present in the first channel104-a, 104-b, or 104-c, but there is a single-photon in first channel104-d, first switch 102-a is configured to shift the single-photon infirst channel 104-d by three channels to first channel 104-d. Anyphotons in the remaining first channels 104 within first switch 102-aare not shifted to first channel 104-a (e.g., they are shifted by threechannels in the same direction or otherwise shifted to channels otherthan first channel 104-a).

In this way, a respective first switch 102 is configured to, whenphoton-availability criteria are met (e.g., when the corresponding setof photon sources 110 produces at least one single-photon), output asingle-photon to a predetermined channel within the respective firstswitch 102. Thus, a respective set of photon sources 110, together witha first switch 102, behaves as a single-photon source (with thepredetermined channel being the output) that has a higher efficiencythan the photon sources 112 comprising the respective set of photonsources 110.

In some embodiments, the predetermined channel for each respective firstswitch 102 is coupled with a different second switch 106. For example,the predetermined channel for first switch 102-a is first channel 104-a,coupled with second channel 108-a of second switch 106-a; thepredetermined channel for first switch 102-b is first channel 104-f,coupled with second channel 108-f of second switch 106-b; thepredetermined channel for first switch 102-c is first channel 104-k,coupled with second channel 108-k of second switch 106-c; and thepredetermined channel for first switch 102-d is first channel 104-p,coupled with second channel 108-p of second switch 106-d.

FIG. 2C shows device 100 at a third time that is after the second time.As best seen by comparing FIG. 2C with FIG. 2B, in FIG. 2C, the photonsin each first switch 102 have been shifted to the predetermined channelfor the first switch 102, using the examples of predetermined channelsdescribed above. In this example, the photon in first switch 102-a hasbeen shifted up by two channels (or, equivalently, shifted down by twochannels under a cyclic permutation), the photon in first switch 102-bhas been shifted down by one channel (or, equivalently, shifted up bythree channels under a cyclic permutation), the photon in first switch102-c has been shifted by zero channels (e.g., maintained in therespective channel), and the photon in first switch 102-d has beenshifted down by three channels (or, equivalently, shifted up by onechannel under a cyclic permutation). In some embodiments, shifting thephotons includes cyclically permuting the photons across the firstchannels 104. By shifting the photons in this manner, first switches 102output the photons onto a respective first channel 104 (e.g., that maybe different from the first channel 104 on which the photon started).

FIG. 2D shows device 100 at a fourth time that is after the third time.At the fourth time, the photons output by the first switches 102 havetraveled down the corresponding first channels 104 (e.g., thepredetermined first channels 104) to the second channels 108, to thecorresponding second switches 106. For example, the first photon outputby first switch 102-a onto first channel 104-a has traveled down secondchannel 108-a to second switch 106-a; the second photon output by firstswitch 102-b onto first channel 104-f has traveled down second channel108-f to second switch 106-b; the third photon output by first switch102-c onto first channel 104-k has traveled down second channel 108-k tosecond switch 106-c; and the fourth photon output by first switch 102-donto first channel 104-p has traveled down second channel 108-p tosecond switch 106-d.

Each second switch 106 is configured to shift photons in the pluralityof second channels by zero or more channels, based on configurationinformation provided to the second switch. In some embodiments, eachsecond switch 106 is configured to shift photons to a predeterminedchannel of the second channels 108 within the second switch 106. In someembodiments, the configuration information indicates a desired set 114of device output terminals 116. As most easily seen by comparing FIG. 2Dand FIG. 2E (which shows device 100 at a sixth time that is after thefifth time), in this example, the configuration information causes eachsecond switch 106 to shift photons to a second channel 108 that iscoupled with a device output terminal 116 in the desired set 114-d ofdevice output terminals 116. Alternatively, when the photons are neededat the set 114-a (or 114-b, or 114-c) of device output terminals 116,the configuration information causes the second switches 106 to outputphotons to the set 114-a of device output terminals 116 (via thecorresponding second channels 108).

In this example, the photon in second switch 106-a has been shifted downby four channels (or, equivalently, shifted up by one channel under acyclic permutation), the photon in second switch 106-b has been shifteddown by three channels (or, equivalently, shifted up by two channelsunder a cyclic permutation), the photon in second switch 106-c has beenshifted down by one channel (or, equivalently, shifted up by threechannels under a cyclic permutation), and the photon in second switch106-d has been shifted by zero channels (e.g., maintained in therespective channel). By shifting the photons in this manner, secondswitches 102 output the photons onto a respective set 114 of deviceoutput terminals 116.

FIG. 2F shows device 100 at a seventh time that is after the sixth time.At the seventh time, the photons have traveled down second channels 108and arrived at the selected set 114-d of device output terminals 116.

Advantageously the dual layer (including the group of first switches asthe first layer and the group of second switches as the second layer)switch fabric described above provides for a photon source that that canroute n non-deterministically generated single photons from any set of ninput channels of the switch fabric to another predetermined set of noutput terminals of the switch. Also advantageously, the switch fabricis constant depth for any number of photon sources, i.e., no matter howlarge the number of photon sources used (and correspondingly, the numberof output terminal used) each photon only passes through a fixed number(e.g., 2) switches before exiting at an output channel that is a memberof the group of predetermined output channels. For example, scaling thesize of the system to include 10, 100, 1,000, 10,000, etc.,probabilistic photon sources (and correspondingly to include 10, 100,1,000, etc., output channels) requires scaling the number of sources,channels, and connections, but not the number of layers in the switchfabric, hence the arrangement is constant depth. Having a fixed depthswitching fabric is important for many applications because photonicswitches can be lossy devices (lead to loss of photons throughabsorption, etc.) and thus the number of switches in any single photonrouting scheme should be minimized if the device is to have a highefficiency. Also, each switch consumes power so minimizing the number ofswitches also minimizes the power consumption of the device. Such a duallayer constant depth switch fabric can be used to provide a multiphotonsource that emits n photons on n predetermined output channels with anincreased probability (approaching 1 for large numbers of inputsources), even if the input sources themselves are non-deterministicphoton sources, e.g., heralded photon sources from some process such asspontaneous four wave mixing, spontaneous parametric down-conversion,and the like. Such a source of n photons can be extremely useful as alow loss, low power, near deterministic source of n photons.

FIGS. 3A-3B are schematic diagrams of various photon switches 300 inaccordance with some embodiments.

FIG. 3A is a schematic diagram illustrating a two-channel switch 300-ain accordance with some embodiments. To aid in understanding,two-channel switch 300-a is shown and described within the context of alarger section 302-a of photonic and electronic componentry.

In some embodiments, two-channel switch 300-a operates (or acts) onoptical modes. An optical mode is defined by a set of physical degreesof freedom of a photon. In some embodiments, an optical mode is definedby a set of all physical degrees of freedom of the photon. In someembodiments, an optical mode is defined by specifying, for a photon: afrequency, a spatial extent (e.g., which channel or superposition ofchannels the photon is localized within), an associated direction ofspatial propagation (e.g., a direction along the channel orsuperposition of channels the photon is travelling), a polarization(e.g., of the photons electric and/or magnetic fields), temporal extent,and an orbital angular momentum (e.g., a direction of the photon'sspin).

Two-channel switch 300-a includes channels 304-a and 304-b (e.g.,analogous to first channels 104, FIGS. 1A-1B) that are coupled withphoton sources 312-a and 312-b (e.g., analogous to photon sources 112,FIGS. 1A-1B). In some embodiments, photon sources 312 include opticalapparatuses for producing heralded single-photons as well as theircorresponding heralding photons. In some circumstances, one photon ofthe pair of photons is used as the outputted photon, while the other isused to “herald” the arrival of the single-photon (e.g., the heraldingphoton is destroyed in the process). In some embodiments, the heraldingphotons are transferred on channels 314-a and 314-b to photon detectors320-a and 320-b, respectively.

A phase selector 316-a selects phases for switch 300-a, as describedbelow, in accordance with a determination of which photon detectors 320detected photons. In some embodiments, phase selector 316-a includesclassical computing circuitry (e.g., non-quantum computing circuitry).For example, in some embodiments, phase selector 316-a includes one ormore application-specific integrated circuits (ASIC) and memory. In someembodiments, the memory stores instructions for selecting phases inaccordance with a determination of which photon detectors 320 detectedphotons. In some embodiments, the memory stores a look-up table storingpredefined phases for different photon detection configurations.

In some embodiments, detectors 320 are coupled to a digital logic module(e.g., which may be implemented as field programmable digital logicusing, for example, a field programmable gate array (FPGA) or an on-chiphard-wired circuit, such as an application specific integrated circuit(ASIC)). Alternatively, in some embodiments, the detectors 320 arecoupled to an off-chip classical computer. In some embodiments, thedigital logic module and/or the classical computer receives informationfrom each detector 320 indicating whether the detector 320 detected aphoton (and optionally how many). Stated another way, the digital logicmodule and/or the classical computer receives the detection pattern fora detection operation from the detectors 320 (e.g., in the form ofanalog detection signals). The digital logic module and/or the classicalcomputer executes logic that configures a set of phase shifters to causea switching of photons to one or more of the output channels.

In some embodiments, photon detectors 320 are capable of resolving anumber of photons (e.g., distinguishing between a shot in which a photonsource 312 produced a single-photon versus a shot in which the photonsource 312 produced two photons). In some embodiments, photon detectors320 are not capable of resolving the number of photons but are onlycapable of resolving whether photons are detected (e.g., photondetectors 320 produce a binary output indicative of whether photons aredetected).

In some circumstances, production of two photons by a single-photonsource represents a defect in the photon production or a failure of thephoton production. In some embodiments, however, when photon detectors320 are not able to resolve the number of photons detected, theprobability of producing two or more photons is sufficiently low (e.g.,less than 1% or less than 5%) that detection of any number of photons isassociated with production of a single-photon (e.g., considered asuccess). In some embodiments, defect or failure associated with asingle-photon source producing two or more photons is handleddownstream. For example, the downstream processes and error correctionare robust enough to handle a certain number of two-photon productions(or other defects). In some embodiments, when the photon production is afailure (e.g., does not meet computational criteria based on downstreamneeds), the failure is dealt with later (e.g., by attempting adownstream computation and allowing the computation to fail because theshot was not good enough, in which case the entire process is repeateduntil the computation is successful).

To allow time for the heralding photon detection (e.g., by photondetectors 320), phase selection (e.g., by phase selector 316-a), andconfiguration of two-channel switch 300-a, in some embodiments, theheralded photons are delayed by delay components 318-a and 318-b (e.g.,delay components are described in greater detail with reference to FIG.5).

The operation of two-channel switch 300-a on an optical mode is nowdescribed (e.g., the action of shifting a photon in one optical modecorresponding to a first channel to a second optical mode correspondingto a second, distinct channel). Assume that a photon is in an opticalmode that is localized within channel 304-a. The photon reaches firstunitary gate 306-a, labeled U, which acts on the photon's state (e.g.,transitions the photon into a superposition state of two optical modes,corresponding to the two channels 304-a and 304-b). The photon in thesuperposition state is then acted on by a set of phase shifters 308-a,labeled D, followed by a second unitary gate 310-a, labeled V (e.g.,which transitions the photon to an optical mode associated with, i.e.,localized within, channel 304-b. In some embodiments, first unitary gate306-a, the set of phase shifters 308-a, and second unitary gate 310-acomprise a decomposition of a permutation matrix (where the permutationmatrix represents the switch):U D V=Σ  (1)

In some circumstances, two-channel switch 300-a is configured tomaintain a photon in the first channel and maintain a photon in a secondchannel. In such circumstances, the permutation matrix leaves thephotons un-shifted. For example, with reference to FIGS. 1A-1B, whenphoton source 112-a produces a single-photon and first channel 104-a isthe predetermined output channel for first set of photon sources 110-a,first switch 102-a can output a single-photon to first channel 104-a bymaintaining the single-photon in first channel 104-a. Thus, whentwo-channel switch 300-a is configured to maintain a photon in the firstchannel and maintain a photon in a second channel, first unitary gate306-a, the set of phase shifters 308-a, and second unitary gate 310-aare configured to satisfy the following conditions.U D V|A

=|A

  (1)U D V|B

=|B

  (2)where |A

is an optical mode localized in channel 304-a, |B

is an optical mode localized in channel 304-b, U, D, V, are 2×2matrices, of which U and V represent the first unitary gate and secondunitary gate, respectively, and D is a set of phase shifters (e.g., adiagonal matrix where each diagonal entry corresponds to a phase shiftfor a photon in a respective channel).

In some circumstances, two-channel switch 300-a is configured to shift aphoton in channel 304-a by one channel to channel 304-b and shift aphoton in channel 304-b by one channel (e.g., cyclically) to channel304-a. In such circumstances, first unitary gate 306-a, the set of phaseshifters 308-a, and second unitary gate 310-a are configured to satisfythe following conditions.U D V|A

=|B

  (3)U D V|B

=|A

  (4)

In some embodiments, the configuration of two-channel switch 300-a isperformed by selecting the phase shifters and leaving unitary gates Uand V unchanged. In some embodiments, the unitary matrices U and V arephysically embodied by a combination of beam-splitters and phaseshifters. For example, in some embodiments, two-channel switch 300-a isa Mach-Zehnder interferometer (MZI) switch.

More generally, in some embodiments, a switch can be represented as afirst unitary matrix U, a first set of phase shifters D₁, a secondunitary matrix W, a second set of phase shifters D₂, and a third unitarymatrix V that comprise a decomposition of a permutation matrix Σ (wherethe permutation matrix represents the switch):U×D ₁ ×W×D ₂ ×V=Σ  (5)

Between unitary gates U and V, the photon need not be in a localizedoptical mode (e.g., an optical mode localized to one of channel 304-a orchannel 304-b), but instead may be in a superposition optical mode.However, a photon localized in one of channel 304-a or channel 304-b,before application of the switch (e.g., before application of thepermutation matrix Σ) will be localized in one of channel 304-a orchannel 304-b after application of the switch.

More generally still, in some embodiments, a switch can be representedas a set of N unitary transformations and a set of N−1 phase shifters.

Returning to the two-channel switch 300-a example with two unitary gatesU and V separated by a single set of phase shifters (FIG. 3A), in someembodiments, U and V are both Hadamard gates:

$\begin{matrix}{U = {V = {H = {\frac{1}{\sqrt{2}}\begin{pmatrix}1 & 1 \\1 & {- 1}\end{pmatrix}}}}} & (6)\end{matrix}$

In this basis,

$\begin{matrix}{\left. A \right\rangle = \begin{pmatrix}1 \\0\end{pmatrix}} & (7) \\{\left. B \right\rangle = \begin{pmatrix}0 \\1\end{pmatrix}} & (8) \\{{H\left. A \right\rangle} = {\frac{1}{\sqrt{2}}\left( {\left. A \right\rangle + \left. B \right\rangle} \right)}} & (9) \\{{H\left. B \right\rangle} = {\frac{1}{\sqrt{2}}\left( {\left. A \right\rangle - \left. B \right\rangle} \right)}} & (10)\end{matrix}$

To maintain a photon in each of channel 304-a and channel 304-b, theconfiguration information is used to set the phase shifts of the set ofphase shifters 308-a, labeled D, is configured to apply zero phase shiftto photons in each of channel 304-a and channel 304-b, under which D isa 2×2 identity matrix.

$\begin{matrix}{D = \begin{pmatrix}1 & 0 \\0 & 1\end{pmatrix}} & (11)\end{matrix}$

Thus, a photon in channel 304-a (represented by its state|A

) is maintained in channel 304-a:

$\begin{matrix}\begin{matrix}{{\Sigma\left. A \right\rangle} = {{UDV}\left. A \right\rangle}} \\{= {\frac{1}{\sqrt{2}}\left( {{{UD}\left. A \right\rangle} + {{UD}\left. B \right\rangle}} \right)}} \\{= {\frac{1}{\sqrt{2}}\left( {{U\left. A \right\rangle} + {U\left. B \right\rangle}} \right)}} \\{= {\frac{1}{2}\left( {\left. A \right\rangle + \left. B \right\rangle + \left. A \right\rangle - \left. B \right\rangle} \right)}} \\{= \left. A \right\rangle}\end{matrix} & (12)\end{matrix}$

And, a photon in channel 304-b (represented by its state |B

) is maintained in channel 304-b:

$\begin{matrix}\begin{matrix}{{\Sigma\left. B \right\rangle} = {{UDV}\left. B \right\rangle}} \\{= {\frac{1}{\sqrt{2}}\left( {{{UD}\left. A \right\rangle} - {{UD}\left. B \right\rangle}} \right)}} \\{= {\frac{1}{\sqrt{2}}\left( {{U\left. A \right\rangle} - {U\left. B \right\rangle}} \right)}} \\{= {\frac{1}{2}\left( {\left. A \right\rangle + \left. B \right\rangle - \left. A \right\rangle + \left. B \right\rangle} \right)}} \\{= \left. B \right\rangle}\end{matrix} & (13)\end{matrix}$

To cyclically shift a photon in each of channel 304-a and channel 304-bby one channel, the set of phase shifters 308-a, labeled D, isconfigured to apply zero phase shift to photons in channel 304-a and aphase shift of π to photons in channel 304-b, under which D is thefollowing 2×2 matrix.

$\begin{matrix}{D = \begin{pmatrix}1 & 0 \\0 & {- 1}\end{pmatrix}} & (14)\end{matrix}$

Thus, a photon in channel 304-a (represented by its initial state |A

) is shifted by one channel to channel 304-b (represented by its finalstate |B

):

$\begin{matrix}\begin{matrix}{{\Sigma\left. A \right\rangle} = {{UDV}\left. A \right\rangle}} \\{= {\frac{1}{\sqrt{2}}\left( {{{UD}\left. A \right\rangle} - {{UD}\left. B \right\rangle}} \right)}} \\{= {\frac{1}{\sqrt{2}}\left( {{U\left. A \right\rangle} - {U\left. B \right\rangle}} \right)}} \\{= {\frac{1}{2}\left( {\left. A \right\rangle + \left. B \right\rangle - \left. A \right\rangle + \left. B \right\rangle} \right)}} \\{= \left. B \right\rangle}\end{matrix} & (15)\end{matrix}$

And, a photon in channel 304-b (represented by its initial state |B

) is shifted by one channel to channel 304-a (represented by its finalstate |A

):

$\begin{matrix}\begin{matrix}{{\Sigma\left. B \right\rangle} = {{UDV}\left. B \right\rangle}} \\{= {\frac{1}{\sqrt{2}}\left( {{{UD}\left. A \right\rangle} - {{UD}\left. B \right\rangle}} \right)}} \\{= {\frac{1}{\sqrt{2}}\left( {{U\left. A \right\rangle} + {U\left. B \right\rangle}} \right)}} \\{= {\frac{1}{2}\left( {\left. A \right\rangle + \left. B \right\rangle + \left. A \right\rangle - \left. B \right\rangle} \right)}} \\{= \left. A \right\rangle}\end{matrix} & (16)\end{matrix}$

FIG. 3B is a schematic diagram illustrating a four-channel switch 300-b.To aid in understanding, four-channel switch 300-b is shown anddescribed within the context of a larger section 302-b of photonic andelectronic components.

Unless otherwise noted, four-channel switch 300-b is analogous totwo-channel switch 300-a. For example, four-channel switch 300-bincludes channels 304-a through 304-d (e.g., analogous to channels 304,FIG. 3A) that are coupled with photon sources 312-a through 312-d (e.g.,analogous to photon sources 312, FIG. 3A), channels 314-a through 314-d(analogous to channels 314, FIG. 3A) which transfer heralding photons tophoton detectors 320-a through 320-b (analogous to photon detectors 320,FIG. 3A), respectively, and a phase selector 316-b (analogous to phaseselector 316-a, FIG. 3A, except that phase selector 316-b selects a 4×4matrix of phases). To allow time for the heralding photon detection(e.g., by photon detectors 320), phase selection (e.g., by phaseselector 316-b), and configuration of four-channel switch 300-b, in someembodiments, the heralded photons are delayed by delay components 318-athrough 318-d (e.g., delay components are described in greater detailwith reference to FIG. 5).

In some embodiments, each first switch 102 and/or each second switch 106(FIGS. 1A-1B) is embodied as four-channel switch 300-b.

Four-channel switch 300-b acts by shifting photons on channels 304(e.g., channel 304-a through 310-d) by zero or more channels. Using thenotation from above, U and V are 4×4 unitary matrices and D is a 4×4diagonal matrix of phase shifts. The matrices U, D, and V comprise adecomposition of a permutation matrix Σ:U D V=Σ  (17)

In some embodiments, four-channel switch 300-b applies a configurablenumber of cyclic permutations to the respective channel numbers ofphoton on channels 304 (and thus permutation matrix Σ applies aconfigurable permutation that is configured, e.g., by selectingappropriate values of the phase shifts in matrix D). For example, whenfour-channel switch 300-b is configured to shift photons down by zerochannels, the following Equations represent the result of the action offour-channel switch 300-b on photons within the respective channels:Σ|A

=|A

Σ|B

=|B

Σ|C

=|C

Σ|D

=|D

  (18)

In the Equations above, |A

represents a photon on channel 304-a; |B

represents a photon on channel 304-b; |C

represents a photon on channel 304-c; and |D

represents a photon on channel 304-d. When four-channel switch 300-b isconfigured to shift photons down by one channel, the following Equationsrepresent the result of the action of four-channel switch 300-b onphotons within the respective channels:Σ|A

=|B

Σ|B

=|C

Σ|C

=|D

Σ|D

=|A

  (19)

When four-channel switch 300-b is configured to shift photons down bytwo channels, the following Equations represent the result of the actionof four-channel switch 300-b on photons within the respective channels:Σ|A

=|C

Σ|B

=|D

Σ|C

=|A

Σ|D

=|B

  (20)

When four-channel switch 300-b is configured to shift photons down bythree channels, the following Equations represent the result of theaction of four-channel switch 300-b on photons within the respectivechannels:Σ|A

=|D

Σ|B

=|A

Σ|C

=|B

Σ|D

=|C

  (21)

In some embodiments, U and V are generalized N-mode unitary gates. Insome embodiments, U and V are generalized N-mode Hadamard gates (e.g., a4×4 Hadamard gate is a 4-mode Hadamard gate).

FIG. 4 is a schematic diagram illustrating an interferometer 400 thatincludes photonics channels in accordance with some embodiments. To thatend, interferometer 400 includes photonic channels 404 (e.g., channels404-a through 404-d). In some embodiments, channels 404 are analogous tochannels 104 and channels 108 (FIGS. 1A-1B). In some embodiments,channels 404 are analogous to channels 304 (FIGS. 3A-3B). In someembodiments, interferometer 400 is a component in a switch (e.g., aswitch 102, 106, FIGS. 1A-1B, a switch 300, FIGS. 3A-3B). In someembodiments, interferometer 400 is a component of a unitary gate (e.g.,a Hadamard gate, as described with respect to FIGS. 3A-3B).

Interferometer 400 includes proximity regions 406 (e.g., proximityregions 406-a through 406-c), where pairs of respective photonicschannels 404 are brought close to one another (e.g., within a wavelengthof a photon). The proximity regions 406 act as beam splitters. Thesebeam splitters can be used to spread the quantum state of a singlephoton (originally localized in a spatial mode defined by one channel)across multiple channels and thus after encountering the beam splitter,the photon can have a non-zero probability amplitude for being detectedin any one of the multiple channels.

For example, the quantum state of a single photon that is localized inchannel 404-a (or having a component in channel 404-a) before enteringproximity region 406-a can be spread among channel 404-a and channel404-b after proximity region 406-a. Likewise, the quantum state of asingle photon that is localized in channel 404-b (or having a componentin channel 404-b) before entering proximity region 406-a can be spreadamong channel 404-a and channel 404-b after proximity region 406-a.

As another example, the quantum state of a single photon that islocalized in channel 404-b (or having a component in channel 404-b)before entering proximity region 406-b can be spread among channel 404-band channel 404-c after proximity region 406-b. Likewise, the quantumstate of a single photon that is localized in channel 404-c (or having acomponent in channel 404-c) before entering proximity region 406-b canbe spread among channel 404-b and channel 404-c after proximity region406-a.

As another example, the quantum state of a photon localized in channel404-c (or having a component in channel 404-c) before entering proximityregion 406-c can be spread among channel 404-c and channel 404-d afterproximity region 406-a. Likewise, the quantum state of a single photonlocalized in channel 404-d (or having a component in channel 404-d)before entering proximity region 406-c can be spread among channel 404-cand channel 404-d after proximity region 406-a.

In some embodiments, interferometer 400 further includes lengtheningregions 408 for delaying a photon. For example, channel 404-a includeslengthening region 408-a and channel 404-d includes lengthening region408-b. Lengthening regions 408 assure that each channel 404 withininterferometer 400 has the same length, so that a photon entering, forexample, channel 404-a and a photon entering, for example, channel 404-bat the same time traverse the interferometer in the same amount of time.

FIG. 5 is a schematic diagram illustrating a photon-delay component 518in accordance with some embodiments. As noted above, in someembodiments, the photon sources provided herein (e.g., photon sources112) produce heralded single-photons. The photon sources also produce acorresponding heralding photon that is detected in order to configurecertain switches in a photon multiplexer (e.g., first switches 102,FIGS. 1A-1B). To allow time for the heralding photon detection (e.g., byphoton detectors 320), phase selection (e.g., by phase selector 316),and configuration of the switches, a heralded photons is delayed bydelay component 518, which includes a geometric lengthening of a channel504 carrying the heralded photon (e.g., the channel forms a spiral thatspirals in and then out).

FIG. 6 is a flowchart of a method 600 for multiplexing photons inaccordance with some embodiments. In some embodiments, the method 600 isperformed at a device that includes photon multiplexing device (e.g.,device 100/120, FIGS. 1A-1B). The photon multiplexing device includes aplurality of first switches (e.g., first switches 102, FIGS. 1A-1B) anda plurality of second switches (e.g., second switches 106, FIGS. 1A-1B).Each first switch in the plurality of first switches includes aplurality of first channels (e.g., first channels 104, FIGS. 1A-1B).Each second switch in the plurality of second switches includes aplurality of second channels (e.g., second channels 108, FIGS. 1A-1B).As described, for example with reference to device 100/120, FIGS. 1A-1B,each second switch includes a respective second channel coupled with arespective first channel from a distinct first switch of the pluralityof first switches.

At a respective first switch of a plurality of first switches (602), thephoton multiplexing device receives (604) one or more photons in theplurality of first channels and shifts (604) the one or more photons inthe plurality of first channels by zero or more channels, based on firstconfiguration information provided to the respective first switch.

In some embodiments, the photon multiplexing device further includes aset of photon sources (e.g., a set of photon sources 110, FIGS. 1A-1B)coupled with the respective first switch of the plurality of firstswitches. At each photon source in the set of photon sources, the photonmultiplexing device attempts to produce a heralded single-photon (e.g.,the one or more photons in the plurality of first channels of therespective first switch comprise the heralded single-photons produced bythe set of photon sources).

In some embodiments, the photon multiplexing device detects (e.g., usingdetectors 320, FIGS. 3A-3B) a set of heralding photons indicative ofwhich photon sources in the set of photon sources successfully produceda heralded photon. The first configuration information is based on whichphoton sources in the set of photon sources successfully produced aheralded photon. For example, with reference to FIG. 2A, the firstconfiguration for information for first switch 102-a indicates thatphoton source 112-c produced a heralded photon.

In some embodiments, shifting, at the respective first switch, the oneor more photons in the plurality of first channels by zero or morechannels includes, when one or more first criteria are met, shifting arespective photon of the one more photons to a predetermined firstchannel of the plurality of first channels of the first switch.

In some embodiments, the first criteria are photon-availabilitycriteria. In some embodiments, the photon-availability criteria are metwhen it is possible to output a single-photon to the predetermined firstchannel within respective first switch (e.g., when at least one photonsource in the set of photon sources 110 has produced a single-photon).In some embodiments, other single-photons produced by the set of photonsources are discarded by the photon multiplexing device.

In some embodiments, the photon multiplexing device configures the firstswitch, based on the first configuration information, to shift therespective photon of the one or more photons to the predetermined firstchannel in the plurality of first channels of the first switch. In someembodiments, configuring the first switch, based on the firstconfiguration information, includes selecting a set of phase shiftvalues for shifting the phases of photons within the respective channelsof the respective first switch (e.g., as described with reference toFIGS. 3A-3B).

At a respective second switch of a plurality of second switches (608),the photon multiplexing device receives (610) one or more photons in theplurality of second channels and shifts (612) the one or more photons inthe plurality of second channels by zero or more channels, based onsecond configuration information provided to the respective secondswitch.

In some embodiments, the plurality of second switches is coupled with aplurality of sets of device output terminals (e.g., sets 114 of deviceoutput terminals 116, FIGS. 1A-1B). The photon multiplexing deviceselects a respective set of device output terminals of the plurality ofsets of device output terminals for outputting photons. The secondconfiguration information is based on the selected set of device outputterminals. Shifting the one or more photons in the plurality of secondchannels by zero or more channels, at a respective second switch of theplurality of second switches, includes shifting a respective photon ofthe one or more photons in the plurality of second channels to arespective second channel coupled with the selected set of device outputterminals.

In some embodiments, the photon multiplexing device configures thesecond switch, based on the second configuration information, to shiftthe respective photon of the one or more photons in the plurality ofsecond channels to the respective second channel coupled with theselected set of device output terminals.

In some embodiments, configuring the second switch, based on the firstconfiguration information, includes selecting a set of phase shiftvalues for shifting the phases of photons within the respective channelsof the respective second switch (e.g., as described with reference toFIGS. 3A-3B).

For example, device 100/120 in FIGS. 1A-1B selects a respective set 114of device output terminals 116 from the plurality of sets 114 of deviceoutput terminals 116. In the example shown in FIGS. 2A-2F, device100/120 selects set 114-d of device output terminals 116. Each of thesecond switches 106 is configured to direct a photon set 114-d of deviceoutput terminals 116 (e.g., via their respective second channels 108).

In some embodiments, when one or more second criteria are met, thephotonic multiplexing device outputs a photon to each device outputterminal in the selected set of device output terminals.

In some embodiments, the one or more second criteria are shot-successcriteria. In some embodiments, photonic multiplexing device includes oris coupled with a plurality of sets of photon sources and the one ormore shot-success criteria include a condition that is met when at leastone photon source in each set of photon sources has produced asingle-photon. In some embodiments, the one or more shot-successcriteria include a condition that is met when at least one photon sourcein each set of photon sources has produced at least one photon. In someembodiments, the one or more shot-success criteria include a conditionthat is met when at least one photon source in a predefined number(e.g., threshold number) of sets of photon sources has produced asingle-photon (e.g., at least three of the four sets of photon sourceshas produced at least one single-photon). In some embodiments, the oneor more success criteria include a condition that is met when at leastone photon source in a predefined number (e.g., threshold number) of setof photon sources has produced at least one photon.

FIGS. 7A-7B shows a high-level conceptual diagram of the 2-layerswitching configuration described above in the embodiments shown in FIG.1-6. More specifically, in this example, the two-layer switchingconfiguration provides for n (4 in this case) photons to be gatheredtogether into n output modes (also 4 in this case). In the example shownin FIGS. 7A-7B, there are 4 blocks of four sources. As before, the firstswitching layer allows cyclic permutations of the photons generatedwithin a block, the second layer allows permutations between the sameposition of each block. One example of how a series of photons from eachlayer could be switched to a specific set of four outputs is shown inFIG. 7A. An alternative representation of this switching configurationthat depicts a clearer visualization of the permutations is shown inFIG. 7B, where initial positions of the photons before the permutationsare shown in dashed circles and final positions after the permutationsare shown as solid circles.

In the example shown, 16 sources are divided into four blocks of four.In the first layer the four modes of each block enter into a cyclicswitch. In the second layer another cyclic switch connects the 1stsource of each block, the 2nd of each block and so on. Advantageously,in this configuration, it is possible to successfully get 4 photons atthe four output modes by arranging one photon into the first position ofits block, one photon into the second position, and so on. Note thatsuch a procedure also works when two photons are initially in the sameblock, as long as each position 1-4 can still be filled. This is clearby looking at the 2nd array in the FIG. 7B. As long as each row containsat least one photon, they can then be pushed to the same column by thesecond layer of switches.

The switch layer visualization shown in FIG. 7B allows for the abilityto clearly illustrate other types of more efficient two-layer switchingconfigurations. In the configurations described above, for an n-by-narray, the largest number of photons that can be retrieved is n.Accordingly, with a source efficiency of 10% at least 10 times moresources than the number of photons we want to retrieve will be needed.However, n becomes large such n-by-n square configurations becomes farmore inefficient.

FIGS. 8A-8B shows another option for a more efficient two-levelswitching configuration, in accordance with some embodiments. In thisexample, the configuration includes m=9 blocks of n=4 sources each. Thegoal of this switching network 800 is to retrieve (i.e., output) as manyphotons as possible in complete blocks of 4. A block size of four ischosen merely for the sake of example and the optimal number ofsources/outputs per block can depend on the source efficiency, andfidelity of the switches. In this case, there are 36 sources in total,divided into 9 blocks of 4. The first layer 815 of switches behavesexactly as described above, coupling groups of photon sources intophoton source groups and allowing any cyclic permutation within a block(i.e., within a column). Accordingly, each switch in the first layer 815of switches only employs connectivity within a block, no inter-blockconnectivity is required.

The second layer of switches 817 employs a sequentially offsettingconfiguration, as shown in FIGS. 8A and 8B. The configuration betweenthe first and second switching layers can be read by sequentiallyconnecting modes across the rows of FIG. 8A. For example, for block 1,the 1^(st), 2^(nd), 3^(rd), and 4^(th) modes (also referred to herein aschannels) are coupled to modes of other blocks as follows: the 1^(st)mode of block 1 is not connected to any second layer switch at all; itis connected directly to an output terminal of the four photon outputgroup 803; the second mode of block 1 is connected to the first mode ofblock 2; the third mode of block 1 is connected to both the second modeof block 2 and the first mode of block 3; the fourth mode of block 1 isconnected to third mode of block 2, the second mode of block 3, and thefirst mode of block 4. As such, the connectivity each of the secondlayer switches (and the number of input and output terminals) increasesas one moves toward the interior of the device. The same pattern thenrepeats for each subsequent block thereby leading to the staggered arraylayout for the switch fabric, as shown in FIG. 8A. In this example thesecond switch layer can be configured so that the system has three setsof four output modes, which are indicated by output mode groups 803,805, and 807. However other numbers of output mode groups are possiblewithout departing from the scope of the present disclosure.

FIG. 8B illustrates a device 800 having an improved second layer switchconfiguration in accordance with some embodiments, like that alreadydescribed above in reference to FIG. 8A and shows that a photonic deviceconfiguration is a one-to-one mapping to the array visualization shownin FIG. 8A. In some embodiments, device 800 is a photonic device. Insome embodiments, device 800 is a hybrid electronic/photonic device(e.g., device 800 includes both electronic and photonic components).Like FIG. 8A, this example, the configuration includes 9 blocks of 4sources each. The goal of this switching network is to retrieve (i.e.,output) as many photons as possible in complete blocks of 4. Like thedevices described above in reference to FIGS. 1A-2F, each block of nsources can include a dedicated n-by-n switch, the collection of whichis referred to herein as the first switch layer. As before, the functionof the first switch layer is to be able to route a photon produced byany photon source within any given block to any output within that sameblock. Accordingly, the first switch layer for a configuration of mblocks, with each block having n sources, can include a set of m n-by-nswitches. Additional details of the operation of the first switch layerwill not be repeated here for the sake of conciseness because thesedetails are set forth above in reference to the above Figures.

In contrast to the second switch layer of the device 100 shown in FIG.1A, which, form blocks, would include another layer of m n-by-nswitches, the connectivity of each switch of the second switch layer hasincreasing connectivity toward the middle of the device (or converselydecreasing connectivity toward the edges of the device). For example,for blocks 1 and 9 in device 900, the first and last modes, respectivelyare not coupled by a any second layer switch at all (they are directlycoupled to output terminals in output group 803 and output group 807respectively. The connection pattern of the remaining switches isstaggered, as described above in reference to FIG. 8A.

Advantageously, the connectivity of each of the second layer switchescan be more local than in other designs. For example, none of the secondlayer switches require coupling between modes of each of the m groups;or stated another way, at least some of the switches are smaller thann-by-n switches. In the example shown in FIG. 8B, the staggered designallows for the outermost switches of the second layer switches to be(n−2)-by-(n−2) switches, followed by (n−1)-by-(n−1) switches, and so on.

Stated yet another way, the staggered arrangement shown in FIGS. 8A-8Bprovides for a switch that allows for the output of as many photons aspossible in complete blocks of n (where n=4 in this case), while alsoallowing for a second switch layer that employs switches having lessthan m-by-m connectivity (which would require 9×9 switches in this caseif it were configured using the design in FIG. 1). For example, in theswitch of FIGS. 8A-8B some of the switches in the second layer are atmost 4×4 switches, and some are even 3×3, and 2×2 switches. Furthermore,the at least two outermost modes are totally unswitched by the secondlayer (e.g., modes 809 and 811 do not have a second layer switch inoptical path). The configuration of FIGS. 8A-8B can be advantageousbecause it requires less connectivity between the switches, e.g., atleast some of the switches in the second switching layer are connectedto less than all m groups, in contrast to device 100.

The terminology used in the description of the various describedembodiments herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used in thedescription of the various described embodiments and the appendedclaims, the singular forms “a”, “an” and “the” are intended to includethe plural forms as well, unless the context clearly indicatesotherwise. It will also be understood that the term “and/or” as usedherein refers to and encompasses any and all possible combinations ofone or more of the associated listed items. It will be furtherunderstood that the terms “includes,” “including,” “comprises,” and/or“comprising,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

It will also be understood that, although the terms first, second, etc.,are, in some instances, used herein to describe various elements, theseelements should not be limited by these terms. These terms are only usedto distinguish one element from another. For example, a first switchcould be termed a second switch, and, similarly, a second switch couldbe termed a first switch, without departing from the scope of thevarious described embodiments. The first switch and the second switchare both switches, but they are not the same switch unless explicitlystated as such.

As used herein, the term “if” is, optionally, construed to mean “when”or “upon” or “in response to determining” or “in response to detecting”or “in accordance with a determination that,” depending on the context.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the scope of the claims to the precise forms disclosed. Manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen in order to best explain theprinciples underlying the claims and their practical applications, tothereby enable others skilled in the art to best use the embodimentswith various modifications as are suited to the particular usescontemplated.

What is claimed is:
 1. A device, comprising: a plurality of photonsources coupled to a plurality of output terminals, wherein theplurality of photon sources are coupled together, by a first switchlayer, into a plurality of photon source groups, wherein the firstswitch layer comprises a plurality of switches; a second switch layercoupled to output terminals of the first switch layer, comprising: aplurality of second layer n-by-n switches; and a plurality of secondlayer l-by-l switches, wherein l is less than n, wherein at least twooutput terminals from two respective photon sources residing within afirst photon source group of the plurality of photon source groups and asecond photon source group of the plurality of photon source groups arecoupled directly to respective output terminals of the device withoutbeing coupled to any intervening second switch from the second switchlayer.
 2. The device of claim 1, further including: a first outermostsecond layer switch that is coupled to at least the first photon sourcegroup and a third photon source group and is a 2-by-2 switch, whereinthe first outermost second layer switch is not coupled to the secondphoton source group; and a second outermost second layer switch that iscoupled to at least the second photon source group and a fourth photonsource group is a 2-by-2 switch, wherein the second outermost secondlayer switch is not coupled to the first photon source group.
 3. Thedevice of claim 1, wherein: the plurality of photon source groupscomprises m photon source groups, and each switch from the second switchlayer has fewer than m inputs and m outputs.
 4. The device of claim 1,wherein the photon sources are heralded single photon sources.
 5. Thedevice of claim 4, including: for each photon source, circuitry todetermine whether the photon source has emitted a photon.
 6. The deviceof claim 1, wherein each switch of the plurality of switches in thefirst switch layer comprises a Mach Zehnder interferometer (MZI) switch.7. The device of claim 1, wherein each switch of the plurality ofswitches in the first switch layer is configured to cyclically permutephotons between modes of the switch.
 8. The device of claim 1, whereineach switch of the plurality of switches in the second switch layercomprises a Mach Zehnder interferometer (MZI) switch.